1. Field of the Invention
The present invention relates to an electroacoustic transducer for converting an electric signal applied thereto into sound.
2. Description of the Prior Art
An electroacoustic transducer is a means for converting an electric signal applied thereto into sound. The electroacoustic transducer can produce an acoustic output in response to an input electric signal. Accordingly, the electroacoustic transducer can be employed by electronic devices, etc. as a sounding means such as a buzzer.
FIGS. 14 and 15 show a prior art electromagnetic type electroacoustic transducer. The electroacoustic transducer includes a housing 102 formed of synthetic resins.
The housing 102 houses therein a base portion 104 and a magnetic driving portion 105. The base portion 104 serves as a supporting member for the magnetic driving portion 105 as well as a closing means for an opening of the housing 102. The base portion 104 comprises seven pieces of five kinds, namely, a core 106, a base 108, a base plate 110, terminals 112 and 114 and caulking pins 116 and 118. The base plate 110 and the base 108 are fixed and double-structured by the core 106 which is penetrated through the two members and caulked at the rear end. The terminals 112 and 114 are respectively fixed to the base plate 110 by the caulking pins 116 and 118.
The core 106 constitutes a part of the magnetic driving portion 105. A coil 120 is wound around the periphery of the core 106 and has leads 122 and 124 which are connected individually to the terminals 112 and 114. An annular magnet 126 is provided around the peripheral portion of the coil 120. The magnet 126 is fixed to the base 108.
The housing 102 has a stepped portion 127 provided at the inner wall thereof which defines a small diameter portion at the front side thereof and a large diameter portion at the rear side thereof. An interval holding ring 128 is disposed between the stepped portion 127 and the base 108. The interval holding ring 128 is a means for supporting a diaphragm 130 while holding the interval between the diaphragm 130 and the end surface of the core 106. That is, the interval holding ring 128 has a receiving surface at the top portion thereof on which the diaphragm 130 is placed. The diaphragm 130 is spaced away from the end surface of the core 106 with a gap 132 which permits a mechanical vibration of the diaphragm 130. A magnetic piece 134 is fixed to the center of the upper surface of the diaphragm 130.
A resonant space 136 is provided inside the housing 102 for producing resonant vibration in response to the vibration of the diaphragm 130. A cylindrical sound emitting hole 138 is formed at the front side of the housing 102. The resonant space 136 is open to the atmosphere through the sound emitting hole 138.
When an electric signal having an appropriate level is applied between the terminals 112 and 114, the coil 120 is energized by the electric signal, which causes the diaphragm 130 to vibrate vertically, namely, upward and downward in FIG. 14. The vibration of the diaphragm 130 depends on amplitude and frequency of the electric signal to be applied thereto. As a result, sound is produced by the vibration of the diaphragm 130 and the resultant sound causes resonant space 136 to produce resonant vibration and it is thereafter discharged from the sound emitting hole 138 to the atmosphere. At this time, the base portion 104 serves to keep the rear side of the diaphragm 130 airtight, which contributes to a predetermined sound characteristics in the course of production of sound.
According to such an electroacoustic transducer, the base portion 104 occupies a very large volume of the housing 102, which prevents the miniaturization and the lightening of the electroacoustic transducer. The core 106 has been conventionally shortened in order to miniaturize and lighten the electroacoustic transducer. The shortening of the core 106 influences the number and form of windings of the coil 120, which weakens the magnetic field, and in turn decreases the magnetic driving force and the sound pressure.
Although the resonant space 136 occupies a large volume of the housing 102, if it is reduced, the electroacoustic transducer can be miniaturized. However, the reduction of the resonant space 136 leads to the deterioration of the resonant effect and is hence not advantageous. Since the resonant space 136 performs an important resonant effect in the course of production of sound, the acoustic characteristics may be much deteriorated by the reduction of the resonant space 136 rather than the decrease of the magnetic driving force due to the shortening of the core 106.